In many ways Mamet has a similar problem of making a lot of characters sound like himself, but he's certainly had his moments of brilliance, and Redbelt is certainly a step up for me.

Tarantino has had moments of speaking for his characters in the past, including Reservoir Dogs but in previous films he's made up for it with a much more solid plot. Death Proof was neither clever or particularly compelling to make up for it.

That said I still enjoyed the film and liked it on its own merits, but Tarantino has built one of the most impressive filmographies of his generation and anything less than Kill Bill/Pulp Fiction will seem like a step down and a bit disappointing.

I believe the full-length version, which also played in-competition at Cannes, is about 25 minutes longer. I say you should try to squeeze it in between your other ongoing missions. Perhaps you'll find the characters to be more well-rounded.

What's unique about this case is that the "extended" edition is the original version. It was always meant to be released separately as such. But Tarantino had to cut it down because otherwise the length of the Grindhouse feature would've gotten out of hand. Just something to keep in mind.